This invention relates to techniques for scrambling and descrambling audio information signals. More particularly, this invention relates to frequency shifting techniques for scrambling and descrambling such signals.
Various techniques have been employed in the past for the purpose of initially scrambling and subsequently descrambling audio information signals. One such technique is known as frequency shifting, wherein the spectrum of original audio information signals is shifted by inversion so that those frequency portions originally lying at the lower end of the audio frequency band are shifted to the upper end while those portions originally lying near the upper end of the band are shifted to the lower end. Typically, this spectral inversion of the original audio information signals is performed prior to recording the signals either alone, or in combination with associated video signals. The purpose of such scrambling, of course, is to prevent unauthorized reproduction or other use. As one commercial example, pre-recorded video cassettes can be rendered unintelligible by scrambling the audio information portion, so that only an authorized subscriber having a proper descrambling unit coupled to the television monitor/receiver can enjoy the program information by descrambling the audio portion.
A major disadvantage with known audio scrambling devices using frequency shifting techniques is the introduction of frequency error upon recording and reproduction of the scrambled signals which adversely affects the descrambling process. In particular, if the scrambled audio signals are recorded on tape there is an unpredictable frequency error which typically occurs in a random fashion upon reproduction of the signals due to mechanical and/or electrical deficiencies in the recording and playback units. During descrambling, this frequency error introduces unwanted components into the recovered signals, resulting in garbled sounds which are annoying at best and unrecognizable at worst.